Kalessaros is truly enormous, even for his kind, at least 1000 metres long and 3000m in wingspan. His scales are black, and constantly exude black smoke which obscures his form.
He has eight great horns on his head, each of which crackles with the contained energy of a Demon Lord. They connect to his skull, which is itself 50m long and constantly belches toxic fumes. His eyes are completely black, but unreflective and dead in appearance.
Each paw can easily shatter the largest tower, and the claws taper to a razor sharp point. Suffused with magic, they shatter the very air as they pass. Around his body, the world twists and burns with magical saturation.
His tail lashes from side to side, causing tremors and knocking down anything it hits. It has a spiked tip, which is 10m long.
His body attracts enormous amounts of lightning and clouds, which surround him and harmlessly turn around his form.
His breath weapon is a stream of flame, lightning, molten metal and magma fifty metres across, magically charged and exploding wherever it hits any object.
Kalessaros the Black lead the force of Great Dragons and their allies which defeated the Alliance of Khongrr and so saved Mythos from destruction.
The book The True Faith of Emiria lists Kalessaros as one of the three beings who threatened to destroy Mythos, and that as such he was exiled from the world.
According to the Ars Draconiae, Kalessaros is a benevolent dragon, only fighting those who threaten to harm the world and seeking to assist the gods. However, he was buried under a hundred miles of rock, within the burning mantle of the world, and chained with unbreakable chains by Neannu. His motions cause earthquakes, and his fire causes volcanoes.
It is known that he had a vulnerability- a 1cm square point just under his brain and inside his mouth. However, reaching it proved impossible for his foes.